Julie Ruder spends her days prosecuting corrupt politicians and other high-profile defendants, including Canadian media baron Conrad Black, who is awaiting sentencing after being convicted of mail fraud and obstruction of justice.

To unwind, she likes to chat with her son Jack, 6, about the new chess moves he's learned and listen to her daughter, Ashley, 4, read from her favorite books. The first hour of the evening after she arrives home "isn't much time in the scheme of the day, but it's very comforting," Ms. Ruder says.

ERIK UNGER

As a recently divorced mother, Ms. Ruder makes a point not to miss that hour, no matter how demanding her work.

"I have to be pretty good at being efficient at my job," she says. "I don't waste a lot of time during the day."

It's always worth the effort. "No matter what happens, if you have a bad day in court or simply wake up on the wrong side of the bed, this always makes me feel better," she says.

As a member of the public corruption unit in the U.S. attorney's office, she builds some of the most complex cases in the federal court system, including one against Mayor Richard M. Daley's former patronage chief Robert Sorich, who was convicted of placing campaign workers on the city payroll in violation of a federal court order.

7 TO 8 P.M.Ms. Ruder loves arriving home to hear what her children did that day. "We talk, we read stories, we sing songs. They put everything in perspective."

Ms. Ruder is known for her modesty and her talent for making legal arguments and investigating cases.

"I don't think she knows how good she is," says Patrick Collins, a former assistant U.S. attorney who supervised her until he left to join the law firm Perkins Coie LLP.

Closing arguments especially have been an opportunity to showcase her talents. She did so in the Sorich and the Black trials, in the latter speaking for seven hours without notes. "Both got rave reviews," Mr. Collins says.

Ms. Ruder's children were part of the reason she became a federal prosecutor in 2004, soon after she made partner at Kirkland & Ellis LLP, where she specialized in commercial litigation.

Once they were born, "it started to become more important to me how I was actually using those hours away from home. Was it making a difference for anyone and was it fulfilling for me?" The short answer was no. "I didn't care so much who won. I didn't have some passionate belief that my clients shouldn't turn over documents as part of litigation," she says.

So, despite a sense of loyalty and affection for the firm, she left.

Ms. Ruder figured out quickly she had made the right call. She had been in the office for two weeks when she was assigned a child pornography case. The file suggested a school principal might be molesting a young boy in his home, but Ms. Ruder was having trouble getting a warrant executed and wasn't sure where to turn.

A colleague sent her to U.S. Attorney Patrick Fitzgerald, who made a call to get the search started. Within hours, the child was rescued and the principal arrested.

"I thought this was amazing," she says. "I couldn't believe this was my job."